Radiator Opening

I am getting ready to paint my 68 Formula S fastback. It is a big block 4 speed car. The stock radiator that came in these cars are so tiny I don't know how they never overheated. I will be building a 572" motor for this car and I want to put in an at least 26" radiator. Does someone make a conversion panel like they do for the E body cars to put in a larger radiator in the A bodies? If not what else can I do before the car is painted?